Cardiac pacemaker

ABSTRACT

A heart stimulator comprises a built-in resonant circuit and an associated rectifier. By means of an oscillator outside the body of the patient, oscillations with a proper frequency may be induced in the resonant circuit, so that the rectifier produces a d.c. voltage. This d.c. voltage is used to reduce the available operating voltage for the final stage of the heart stimulator in order that the safety margin may be checked.

United States Patent [1 1 Van Den Berg [451 Feb. 20, 1973 [54] CARDIACPACEMAKER [56] References Cited [75] Inventor: Janwillem Van Den Berg,Gronin- UNITED STATES PATENTS gen, Netherlands 3,426,748 2/1969 Bowers..l28/4l9 P [73] Assignee: Vitatron N.V., Dieren, Netherlands [22]Filed: April 1971 FOREIGN PATENTS OR APPLICATIONS pp NOJ 132,577 985,7973/1965 Great Britain ..l28/4l9 P Relaed Appficafion Data PrimaryExaminerWilliam E. Kamm AttorneyMartin Kirkpatrick [63] Continuation ofSer. No. 773,439, Nov. 5, 1968, abandoned. 7] [30] Foreign A li ti P iit D t A heart stimulator comprises a built-in resonant circuit and anassociated rectifier. By means of an oscil- NOV. 16, I967 Netherlands..67l5556 later outside the y of the patient, oscillations with a properfrequency may be induced in the resonant [52] U.S. CI. ..128/4l9 P,128/422 circuit so that the rectifier produces a voltage [51] IIIL Cl...A61Il 1/36 This voltage is used to reduce tha available [58] new ofSearch "128/419 419 419 419 operating voltage for the final stage of theheart stimu- 128/421 422 later in order that the safety margin may bechecked.

8 Claims, 1 Drawing Figure R1 R7 i g/ L c3 T04 c1 NA 1 n w D1 R6 Tl (:2A; ll R4 RB KT3 R5 L a CARDIAC PACEMAKER RELATED APPLICATION This caseis a continuation of application of Ser. No. 773,439, filed Nov. 5, i968and now abandoned.

BACKGROUND OF THE INVENTION A heart stimulator is a source of electricimpulses which is implanted by a surgical operation in the body of apatient in order to stimulate the heart function.

The conventional heart stimulators comprise an impulse generator such asa multivibrator and a final or power amplifier transmitting thegenerated impulses to a ventricular heart electrode. Additional circuitsmay be provided to synchronize the impulse generator with the heart beatand to suppress the impulses during certain critical intervals of theheart beat period. The heart stimulator is entirely transistorized andis energized by a built-in battery.

A certain minimum amplitude of the impulses delivered by the final stageof the heart stimulator is required in order that the heart function maybe properly stimulated. This minimum amplitude will be indicatedhereinafter as the stimulation threshold. Of course, a certain safetymargin is desirable, so that the actual amplitude of the impulses mustexceed the stimulation threshold.

The safety margin may be readily checked during the implantingoperation. However, the stimulation threshold may change after theimplantation of the heart stimulator, for instance if the heartelectrode is displaced or surrounded by connective tissue. In such acase it is still possible to check whether the heart function of thepatient is properly stimulated, but it is no longer possible to checkwhether the amplitude of the impulses supplied to the heart electrodeexceeds the threshold value with such an amount that there is asufficient safety margin. It is an object of the invention to removethis disadvantage and to provide means for checking the available safetymargin at any time.

SUMMARY OF THE INVENTION According to the invention the heart stimulatorcomprises a resonant circuit tuned to a predetermined frequency, arectifier connected with said resonant circuit and an output impedancefor said rectifier inserted in the output circuit of the final amplifierof the pacemaker with such a polarity that the dc. voltage generatedacross the output impedance by the rectifier opposes the batteryvoltage. Thus, the operating voltage available for the final amplifiermay be reduced by inducing oscillations having said predeterminedfrequency in said resonant circuit from the outside.

The available operating voltage for the final stage of the heartstimulator may now be reduced at any desired time to such an extent thatthe heart is no longer properly stimulated. By means of electrodesarranged on the skin of the patient it is possible to measure theimpulse amplitude at which the heart stimulation ceases as well as theimpulse amplitude occurring when no outside oscillations are induced.These two measurements provide a relative stimulation threshold whichprovides information about the available safety margin.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows the circuit diagramof a preferred embodiment of the heart stimulator according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The heart stimulator shown inthe drawing comprises a multivibrator serving as an impulse generator.This multivibrator is equipped with an npn transistor T1 and a pnptransistor T2 cooperating with a plurality of resistors R1, R2, R3, R4and R5 and with a condenser C1. The emitter of transistor T1 is directlyconnected with a conductor Al leading to the negative pole of a battery.The collector of transistor T1 is connected through resistor R1 with aconductor A2 leading to a source of reference potential, such as thepositive pole of the battery; in addition the collector of transistor T1is connected through condenser Cl with the base of transistor T2. Theemitter of transistor T2 is directly connected with conductor A2, whilethe collector is connected with conductor Al through a series circuitincluding resistors R4 and RS. In addition the collector of transistorT2 is connected with the base of transistor T1 through resistor R2. Thebase of transistor T2 is connected with conductor A1 through resistorR3.

The operation of the multivibrator is as follows. When transistor T2 isrendered conductive its collector voltage increases to such a value thattransistor T1 is likewise rendered conductive through resistor R2. Thecollector of transistor T1 is then substantially at the potential ofconductor A1. Condenser C1 is now charged through the diode constitutedby the emitter and the base of transistor T2 until the voltage acrosscondenser Cl is substantially equal to the battery voltage. As soon asthis condition has been reached transistor T2 is cut off, wherebytransistor T1 is likewise cut off through resistor R2. The collectorpotential of transistor T1 is now increased to a value substantiallyequal to the potential of conductor A2, whereby the base potential oftransistor T2 is increased to about twice this value. Condenser C1 isnow discharged through resistor R3; as soon as the voltage acrosscondenser Cl has sufficiently decreased transistor T2 is again renderedconductive and the above described operation is repeated. The timeconstant of the circuit formed by condenser C1 and resistor R3 isrelatively large, for instance of the order of one second, whereascondenser C1 is relatively quickly charged through the emitter-base-pathof transistor T2, so that the multivibrator generates short impulseswith relatively long intervals between them.

The impulses or pulses generated by the multivibrator occur acrossresistor R5, so that they are supplied to the final amplifier stagewhich comprises an npn transistor T3 and two series connected resistorsR6 and R7. The output impulses of transistor T3 are supplied through acondenser C2 to a conductor A3 leading to g the ventricular heartelectrode. A conductor A4 connected with conductor A1 leads to anindifferent electrode which may consist, for instance, of a conductivesurface on the outside of the casing of the heart stimulator.

The emitter of transistor T3 is directly connected with conductor All,while the collector is connected with conductor A2 through the seriesconnected resistors R6 and R7 and with conductor A3 through condenserC2. The base of transistor T3 is connected with the junction ofresistors R4 and R5. Transistor T3 is normally cut off and is renderedconductive by each impulse occurring across resistor R5. Thus,transistor T3 operates as a switch. The circuit as described has theadvantage that the battery is only loaded during the occurrence of animpulse, so that it has a long life.

According to the invention the heart stimulator com prises a resonantcircuit consisting of an inductance L and a condenser C3 connected inparallel with the inductance. This resonant circuit means is connectedwith a diode D1 of which the output impedance includes resistor R7 and acondenser C4 in parallel with resistor R7. The polarity of diode D1 hasbeen selected in such manner that the dc. voltage generated acrossresistor R7 by the diode current is opposed to the bat tery voltage.

By means of an oscillator of which the frequency is equal to the tuningfrequency of the circuit L, C3 and which is arranged outside the body ofthe patient oscillations may be induced in the resonant circuit. Theseoscillations are rectified by diode D1, so that a dc. voltage reducingthe operating voltage of transistor T3 occurs across resistor R7. Theamplitude of the impulses supplied to conductor A3 is thereby reduced.

By adjusting the power of the oscillator the dc. voltage across resistorR7 may be varied within wide limits. Thus, the dc voltage acrossresistor R7 may be increased to such an extent that the heart of thepatient is no longer stimulated. The impulse amplitude at which thestimulation ceases is measured by means of electrodes arranged on theskin of the patient; the impulse amplitude during normal operation,i.e., without use of the oscillator is measured in the same manner. Theratio of the values obtained by these measurements is the relativestimulation threshold which may be considered as an indication of theavailable safety margin.

The frequency of the oscillations induced in the resonant circuit mustbe selected in such manner that they have no influence on the heartfunctions. The frequency may, for instance, be of the order of 2500cycles per second.

Although the invention has been described hereinbefore with reference toa specific embodiment the invention is not restricted to this embodimentwhich may be modified in several ways within the scope of the appendedclaims; in particular the heart stimulator may contain additionalcircuits for synchronizing and suppressing the impulses as indicatedhereinbefore.

I claim:

1. A heart stimulator adapted to be incorporated in a human body,comprising a battery, a plurality of transistors supplied by saidbattery, an impulse generator, including at least one of the saidtransistors, output means including another one of the said transistors,an input for said output means, means for supplying the output impulsesof said impulse generator to the input of said output means, a resistorinserted between the collector of the transistor of said output meansand one of the poles of said battery, a direct connection between theemitter of the transistor of said output means and the other pole ofsaid battery, 21 pair of output terminals respectively connected withthe collector and with the emitter of the transistor of said outputmeans, a resonant circuit tuned to a predetermined frequency, rectifiermeans connected with said resonant circuit, and an output impedance forsaid rectifier means inserted in series with said resistor, saidrectifier means being connected to said output impedance for generatinga DC voltage thereacross, opposing the battery voltage, so that theoperating voltage available for said output means is reduced by inducingoscillation in said resonant circuit at said collector from the outside.

2. A heart stimulator adapted to be incorporated in a human body,comprising a battery, an impulse generator, including a firsttransistor, having its emitter directly connected with a first pole ofsaid battery, a first resistor inserted between the collector of saidfirst transistor and the second pole of said battery, a secondtransistor having a conductivity type opposite to that of said firsttransistor, said second transistor having its emitter directly connectedwith said second pole, a second resistor inserted between the base ofsaid first transistor and the collector of said second transistor, athird resistor inserted between the base of said second transistor andsaid first pole, a first condenser inserted between the base of saidsecond transistor and the collector of said first transistor, fourth andfifth resistors connected in series between the collector of said secondtransistor and said first pole, an output means having input and outputcircuits, means for supplying impulses appearing at the junction betweensaid fourth and fifth resistors to the input circuit of said outputmeans, a pair of output terminals included in the output circuit of saidoutput means, a resonant circuit, said resonant circuit being tuned to apredetermined frequency, rectifier means connected with said resonantcircuit and an output impedance for said rectifier means, said outputimpedance being series connected with said battery in the collectorcircuit of said output means, said rectifier means being connected tosaid output impedance for generating a DC voltage thereacross opposingthe battery voltage so that the operating voltage available for saidoutput means may be reduced by inducing the oscillations in saidresonant circuit from the outside.

3. A heart stimulator as claimed in claim 2 wherein said output meanscomprises a third transistor having the same conductivity type as saidfirst transistor, having its emitter directly connected with said firstpole and having its base connected with said junction, and sixth andseventh resistors connected in series between the collector of saidthird transistor and said second pole, said seventh resistor beingincluded in said output impedance.

4. A heart stimulator as claimed in claim 3 further comprising a secondcondenser inserted between the collector of said third transistor andsaid conductor adapted to be connected with a ventricular heartelectrode.

5. A heart stimulator as claimed in claim 3 wherein said resonantcircuit comprises an inductance and a thirdcondenser in parallel withsaid inductance, and wherein said rectifier is inserted between one endof said resonant circuit and one end of said seventh resistor, the otherends of said resonant circuit and said seventh resistor being directlyinterconnected.

6. A heart stimulator as claimed in claim 3 wherein said outputimpedance further comprises a fourth condenser in parallel with saidseventh resistor.

7. In a heart stimulator adapted to be implanted in a living body anddeliver stimulating pulses to the heart thereof, said stimulatorincluding an output stage with a transistor and a first impedanceconnected in the collector circuit of said transistor, that improvementwhich consists of DC voltage generating circuitry including inductivemeans for receiving energy from a source external to said body, a firstcapacitor connected across said inductive means to form a resonantcircuit therewith, a rectifier and a second capacitor connected inseries across said resonant circuit, and a second impedance connectedacross said second capacitor and series connected with said firstimpedance in the collector circuit of said transistor, said circuitrygenerating DC voltage across said second impedance in response to saidexternal source, thereby modifying the DC'operating voltage for saidoutput stage and the amplitude of said stimulating pulses.

8. Apparatus as claimed in claim 7, including a battery connected toprovide power for the collector circuit of said output stage, said DCvoltage generating circuitry being connected with polarity opposing saidbattery.

1. A heart stimulator adapted to be incorporated in a human body,comprising a battery, a plurality of transistors supplied by saidbattery, an impulse generator, including at least one of the saidtransistors, output means including another one of the said transistors,an input for said output means, means for supplying the output impulsesof said impulse generator to the input of said output means, a resistorinserted between the collector of the transistor of said output meansand one of the poles of said battery, a direct connection between theemitter of the transistor of said output means and the other pole ofsaid battery, a pair of output terminals respectively connected with thecollector and with the emitter of the transistor of said output means, aresonant circuit tuned to a predetermined frequency, rectifier meansconnected with said resonant circuit, and an output impedance for saidrectifier means inserted in series with said resistor, said rectifiermeans being connected to said output impedance for generating a DCvoltage thereacross, opposing the battery voltage, so that the operatingvoltage available for said output means is reduced by inducingoscillation in said resonant circuit at said collector from theoutside.
 1. A heart stimulator adapted to be incorporated in a humanbody, comprising a battery, a plurality of transistors supplied by saidbattery, an impulse generator, including at least one of the saidtransistors, output means including another one of the said transistors,an input for said output means, means for supplying the output impulsesof said impulse generator to the input of said output means, a resistorinserted between the collector of the transistor of said output meansand one of the poles of said battery, a direct connection between theemitter of the transistor of said output means and the other pole ofsaid battery, a pair of output terminals respectively connected with thecollector and with the emitter of the transistor of said output means, aresonant circuit tuned to a predetermined frequency, rectifier meansconnected with said resonant circuit, and an output impedance for saidrectifier means inserted in series with said resistor, said rectifiermeans being connected to said output impedance for generating a DCvoltage thereacross, opposing the battery voltage, so that the operatingvoltage available for said output means is reduced by inducingoscillation in said resonant circuit at said collector from the outside.2. A heart stimulator adapted to be incorporated in a human body,comprising a battery, an impulse generator, including a firsttransistor, having its emitter directly connected with a first pole ofsaid battery, a first resistor inserted between the collector of saidfirst transistor and the second pole of said battery, a secondtransistor having a conductivity type opposite to that of said firsttransistor, said second transistor having its emitter directly connectedwith said second pole, a second resistor inserted between the base ofsaid first transistor and the collector of said second transistor, athird resistor inserted between the base of said second transistor andsaid first pole, a first condenser inserted between the base of saidsecond tRansistor and the collector of said first transistor, fourth andfifth resistors connected in series between the collector of said secondtransistor and said first pole, an output means having input and outputcircuits, means for supplying impulses appearing at the junction betweensaid fourth and fifth resistors to the input circuit of said outputmeans, a pair of output terminals included in the output circuit of saidoutput means, a resonant circuit, said resonant circuit being tuned to apredetermined frequency, rectifier means connected with said resonantcircuit and an output impedance for said rectifier means, said outputimpedance being series connected with said battery in the collectorcircuit of said output means, said rectifier means being connected tosaid output impedance for generating a DC voltage thereacross opposingthe battery voltage so that the operating voltage available for saidoutput means may be reduced by inducing the oscillations in saidresonant circuit from the outside.
 3. A heart stimulator as claimed inclaim 2 wherein said output means comprises a third transistor havingthe same conductivity type as said first transistor, having its emitterdirectly connected with said first pole and having its base connectedwith said junction, and sixth and seventh resistors connected in seriesbetween the collector of said third transistor and said second pole,said seventh resistor being included in said output impedance.
 4. Aheart stimulator as claimed in claim 3 further comprising a secondcondenser inserted between the collector of said third transistor andsaid conductor adapted to be connected with a ventricular heartelectrode.
 5. A heart stimulator as claimed in claim 3 wherein saidresonant circuit comprises an inductance and a third condenser inparallel with said inductance, and wherein said rectifier is insertedbetween one end of said resonant circuit and one end of said seventhresistor, the other ends of said resonant circuit and said seventhresistor being directly interconnected.
 6. A heart stimulator as claimedin claim 3 wherein said output impedance further comprises a fourthcondenser in parallel with said seventh resistor.
 7. In a heartstimulator adapted to be implanted in a living body and deliverstimulating pulses to the heart thereof, said stimulator including anoutput stage with a transistor and a first impedance connected in thecollector circuit of said transistor, that improvement which consists ofDC voltage generating circuitry including inductive means for receivingenergy from a source external to said body, a first capacitor connectedacross said inductive means to form a resonant circuit therewith, arectifier and a second capacitor connected in series across saidresonant circuit, and a second impedance connected across said secondcapacitor and series connected with said first impedance in thecollector circuit of said transistor, said circuitry generating DCvoltage across said second impedance in response to said externalsource, thereby modifying the DC operating voltage for said output stageand the amplitude of said stimulating pulses.